From Froomkin:
Colbert was merciless, reserving his most potent zingers for the people in spitting distance: The president who took the nation to war on false pretenses and the press corps that let him do it.
The boozy bonhomie of the annual event is intended to serve as a balm for the often tense relationship between the White House and the reporters who cover it.
Bush largely delivered on his side of the bargain. Colbert delivered something else entirely.